Meteor radar observations of horizontal winds in the mesosphere/lower thermosphere (MLT) at Collm (51°N, 13°E) and Obninsk (55°N, 37°E) from 1980 to date have been used to analyse the seasonal and interannual variability of the quarterdiurnal tide at middle latitudes. The zonal amplitudes show a clear maximum in boreal winter and a weaker one during spring. Amplitudes increase with height, with up to 7 m/s in the lower thermosphere. The meridional amplitudes are weaker, but show a similar seasonal cycle. Amplitudes and phases at Collm and Obninsk are similar, indicating that most of the observed 6-hour oscillation at higher midlatitudes is due to the migrating quarterdiurnal tide. Model results using a mechanistic circulation model qualitatively reproduce the observations. Obninsk amplitudes show an interdecadal variation with smaller values during the 1990s and larger ones during the 2000s. Possible reasons for the interannual variability are discussed.